A known rotary hearth furnace includes an outer wall, an inner wall, and an annular rotary hearth disposed between the outer wall and the inner wall. The rotary hearth generally includes an annular furnace frame, a hearth heat insulation material disposed over the furnace frame, and a refractory disposed on the hearth heat insulation material. Such a rotary hearth is rotated by a mechanism. The rotating mechanism may be, for example, a gear mechanism including a rotating shaft disposed at the lower portion of the hearth, a pinion gear fixed to the rotating shaft so as to be rotated together with the rotating shaft, a rack rail circularly fixed to the bottom of the furnace frame and engaging with the pinion gear, or an apparatus including a track annularly laid on the surface of the hearth and a plurality of driving wheels disposed at the bottom of the furnace frame and running on the track.
Rotary hearth furnaces including such a mechanism can be used for heat treatment of metals, such as steel billet, or heat treatment of combustible wastes. In addition, a process that produces reduced iron from iron oxides in the rotary hearth furnace has come to attention. An example of the process for producing reduced iron in the rotary hearth furnace will now be described with reference to FIG. 6, which shows a schematic structure of the rotary hearth furnace.    (1) Iron oxide powder (iron ore, electric furnace dust, etc.) and carbonaceous reductant powder (coal, coke, etc.) are mixed and agglomerated, thereby forming green-pellets.    (2) The green pellets are heated in a range of temperatures at which combustible components evaporated from the pellets do not ignite. Water adhering to the green pellets is removed by this heating, and thus dried pellets 24 shown in FIG. 6 are produced.    (3) The dried pellets are fed into the rotary hearth furnace 26 by an appropriate charging device 23 shown in FIG. 6 and are spread on the rotary hearth 21 to form a pellet layer with a thickness defined by one or two pellets.    (4) The pellet layer is heated to be reduced by radiation of combustion in a burner 27 provided at an upper portion in the furnace. Thus, the pellet layer is metalized.    (5) The metalized pellets are cooled by a cooler 28. The cooling is performed by, for example, directly jetting a gas onto the pellets, or an indirect technique using a water-cooling jacket. The cooling gives the pellets such a mechanical strength as they can endure being handled while or after they are discharged. The cooled pellets (for example, reduced iron) 25 are discharged to the outside of the furnace by a discharging device 22.    (6) Immediately after the pellets 25 are discharged, the charging device 23 feeds other dried pellets. The sequence of these steps is repeated to produce reduced iron.
Combustible exhaust gas generated in the rotary hearth furnace used for producing reduced iron is drawn from an exhaust gas discharge region located on the circumference of the rotary hearth furnace to an exhaust duct connected to the ceiling of the exhaust gas discharge region, and is discharged to the outside of the system through an exhaust gas treatment apparatus disposed in the downstream from the exhaust duct. However, part of the combustible components remain in the exhaust gas drawn to the exhaust duct from the rotary hearth furnace. This is because the exhaust gas cannot be sufficiently mixed with oxygen in the rotary hearth furnace, so that the combustible components cannot be completely burned.
Accordingly, in general, secondary combustion air is supplied into the furnace so that the combustible gas can be burned with the secondary combustion air to reduce fuel consumption. However, the supply of the secondary combustion air excessively increases the amount of air in the furnace. The excess of the air not only reduces the temperature in the furnace, but also inhibits the reduction reaction or causes reoxidation.
A metal reduction process using an apparatus as shown in FIG. 7 has been known for controlling the secondary combustion. FIG. 7 shows a horizontal section of this apparatus. The apparatus includes a rotary hearth furnace having a heating zone 30 and a reducing zone 40, a gas analyzer 31, and an air intake means 34. The gas analyzer 31 samples the gas from the heating zone 30 of the furnace and measures the O2 or CO concentration in the gas. The air intake means 34 introduces air into the heating zone 30 according to the concentration measured by the gas analyzer 31, so that the unburned combustibles generated in the reducing zone 40 are burned.
This apparatus, however, cannot sufficiently mix and agitate the secondary combustion air and the combustible gas or ensure residence time sufficient to burn the combustible gas, even if a sufficient amount of air for the secondary combustion is introduced to the heating zone 30. Accordingly, the unburned combustibles may not be completely burned in the region where heat of combustion should be used efficiently. In other words, because the known apparatus cannot sufficiently mix or agitate the unburned combustibles and the secondary combustion air in the furnace, the unburned combustibles do not burn completely. Thus, part of the unburned combustibles are drawn to the exhaust duct without being burned. This phenomenon causes not only the decrease of the fuel efficiency of the rotary hearth furnace, but also damages to equipment, such as the exhaust duct, from the combustion of the unburned combustibles in the exhaust duct and the increase of its temperature. Furthermore, ash and other components contained in the exhaust gas with a high temperature may be melted and adhere to the duct, thus clogging the duct.